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WIP (Not ready): STYLE: Replace once_flag of ThreadPoolGlobals with static local variable #4171

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WIP (Not ready): STYLE: Replace once_flag of ThreadPoolGlobals with static local variable #4171

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12 changes: 5 additions & 7 deletions Modules/Core/Common/src/itkThreadPool.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -40,9 +40,6 @@ struct ThreadPoolGlobals
// To lock on the various internal variables.
std::mutex m_Mutex;

// To allow singleton creation of ThreadPool.
std::once_flag m_ThreadPoolOnceFlag;

// The singleton instance of ThreadPool.
ThreadPool::Pointer m_ThreadPoolInstance;

Expand Down Expand Up @@ -75,8 +72,8 @@ ThreadPool::GetInstance()
// initialized.
itkInitGlobalsMacro(PimplGlobals);

// Create a singleton ThreadPool.
std::call_once(m_PimplGlobals->m_ThreadPoolOnceFlag, []() {
// Create a ThreadPool singleton as part of the initialization of a static local variable, to ensure thread-safety.
[[maybe_unused]] static const volatile bool isCalledOnce = [] {
m_PimplGlobals->m_ThreadPoolInstance = ObjectFactory<Self>::Create();
if (m_PimplGlobals->m_ThreadPoolInstance.IsNull())
{
Expand All @@ -85,7 +82,8 @@ ThreadPool::GetInstance()
#if defined(ITK_USE_PTHREADS)
pthread_atfork(ThreadPool::PrepareForFork, ThreadPool::ResumeFromFork, ThreadPool::ResumeFromFork);
#endif
});
return true;
}();

return m_PimplGlobals->m_ThreadPoolInstance;
}
Expand All @@ -107,7 +105,7 @@ ThreadPool::SetDoNotWaitForThreads(bool doNotWaitForThreads)
ThreadPool::ThreadPool()
{
// m_PimplGlobals->m_Mutex not needed to be acquired here because construction only occurs via GetInstance which is
// protected by call_once.
// protected by the thread-safe initialization of a static local variable, `isCalledOnce`.

m_PimplGlobals->m_ThreadPoolInstance = this; // threads need this
m_PimplGlobals->m_ThreadPoolInstance->UnRegister(); // Remove extra reference
Expand Down